Summary:
Previously OID/OIL required exactly one configuration file. This change makes it so you can supply 0 or more configuration files. 0 is useful if you have pre-generated the cache or use some sort of remote generation system. 1 is useful for the common case, where you have a configuration file that describes your entire source and use just that. More are useful if you have supplemental bits of config you wish to apply/override - see the changes to the integration test framework where we do exactly this.
Test Plan:
This isn't super well tested. It works for the test cases which add features via the config or enable `codegen.ignore`.
- CI
Reviewed By: ajor
Differential Revision: D49758032
Pulled By: JakeHillion
